Re: [CAD_CAM_EDM_DRO] z axis drive question

Dave Fisher wrote:

>I am in the process of building a Z axis for an X Y table I am converting into
>a CNC router. I plan to use Artsoft mach2 software with Rutex servo amps to drive the system.
>
>Can anyone give me any more info on the following:
>
>I have a quality 25 mm thick ball nut screw cut to 5 tpi - I have a choice of rotary
>encoders from 400 to 2000 steps per rev. I intend to drive the screw
>direct from my DC servo motor shaft.
>
>Not being that familiar with servo systems (the amps haven't arrived yet !) I am concerned
>that when I turn the ball screw by hand, a mere fraction of a turn seems to move the ball nut a long way !
>Can a DC servo system with my proposed setup resolve enough resolution to move the ball a few thou,
>typically for fine, milling depth cuts for finishing etc.
>
>

If the screw is really 5 TPI, then it should move .200" = 5.08 mm each turn.
If you have a shaft encoder with 1000 cycle/revolution, that will give 4000
counts/revolution. That is 20,000 counts/inch, or .00005" (50 uInches) per
count, or 0.00127 mm. So, yes, a servo motor can move in incredibly
small increments, and then hold that position.

Jon